1. Field of the Invention
The present invention relates to an improvement of gain flatness of a Raman amplifier using a wavelength multiplexed pump light source in a wavelength-division multiplexing transmission system.
2. Description of the Related Art
An optical fiber using silica as its main component and containing germanium in its core is widely used. It is known that, in a Raman amplifier using this optical fiber, maximum Raman gain value is obtained at a wavelength spaced apart by approximately 100 nm to the long wavelength side from the pump light source wavelength. An attempt has been made, utilizing this phenomenon, to amplify a wavelength-division multiplexing signal (hereinafter referred to as WDM signal) using a plurality of light sources of different wavelengths (hereinafter referred to as wavelength multiplexed pump light sources). To prevent pump lightwave on the longest wavelength side of the wavelength multiplexed pump light source from overlapping signal lightwaves on the shortest wavelength side of the WDM signal, it is necessary for the maximum wavelength band of the wavelength multiplexed pump light source to be approximately 100 nm.
Japanese Patent Application Laid-open No. 2000-98433 discloses that it is necessary to improve the flattening of the Raman gain wavelength property over a wide band in order to effect wavelength-division multiplexing transmission of signal lightwaves, and that it is necessary to take into account the wavelength arrangement of the wavelength multiplexed pump light source.
More specifically, regarding the arrangement of a plurality of pump light sources of different wavelengths, when the wavelength interval between adjacent pump lightwaves is less than 6 nm, it is impossible to secure a margin obtained by adding the band width of a combiner which does not involve an increase in the insertion loss of the combiner due to crosstalk of the combiner when combining pump lightwaves of different wavelengths to the band width of the pump light source. When the wavelength interval exceeds 35 nm, a reduction in gain which is so great as to be unsuitable for wavelength-division multiplexing transmission occurs near the center of the total Raman gain band width of the respective Raman gains generated from adjacent wavelengths, so that it is necessary for the wavelength interval of the pump lightwave to be in the range of 6 nm to 35 nm.
Further, there has been disclosed a technique in which, in order to make the wavelength interval of adjacent pump light sources as small as possible so that the total gain flatness may not increase, the pump lightwave is divided into lightwaves for forward pump and lightwaves for backward pump; for example, the pump light wavelength interval respectively belonging to the forward pump and backward pump is approximately 6 nm, and pump light wavelengths xcex2, xcex4, . . . respectively belonging to the backward pump are arranged between pump light wavelengths xcex1, xcex3, . . . belonging to the forward pump, whereby the pump light wavelength interval of the Raman amplifier is made less than 6 nm to realize a dense pump light wavelength arrangement, making it possible to realize a Raman amplifier so that the difference between maximum and minimum of the Raman gain wavelength property of the Raman amplifier, that is, the gain flatness is so small as to allow dense WDM transmission.
In the Raman amplifier disclosed in Japanese Patent Application Laid-open No. 2000-98433, forward pump and backward pump are treated independently; assuming that the combining position of forward pump lightwave is A and that the combining position of backward pump lightwave is B, the longitudinal section Axe2x86x92B of the optical fiber where stimulated Raman scattering is generated by the forward pump lightwave and the longitudinal section Bxe2x86x92A of the optical fiber where stimulated Raman scattering is generated by the backward pump lightwave are common to each other except for the pump direction.
That is, in the invention as disclosed in Japanese Patent Application Laid-open No. 2000-98433, an amplifier is formed so that gain flatness is so small as to allow dense WDM transmission by using a forward pump light source and a backward pump light source, Raman amplification being effected in the same section in the optical transmission system.
However, any practical Raman amplifier has a certain amount of gain deviation within the gain band even if it is the Raman amplifier with small gain flatness as disclosed in Japanese Patent Application Laid-open No.2000-98433.
That is, regarding the Raman amplifier in which forward pump and backward pump are combined with each other, even in the case of an amplifier with small gain flatness, when Raman amplifiers of the same gain wavelength property are used in a plurality of stages, the wavelength determining the maximum value of the Raman gain wavelength property and the wavelength determining the minimum value thereof are the same in each Raman amplifier, so that each time the amplification stages are passed, the maximum values and minimum values of the Raman gain wavelength property are accumulated, and the deviation of the Raman gain wavelength property increases, with the result that there is a large difference in power between the channels, resulting in a rather poor degree of flatness. For example, when inputting a WDM signal to an optical amplifier using an erbium doped optical fiber (EDF), or after outputting it from the optical amplifier, it is necessary to compensate for the power for each channel by a means like an equalizer.
According to a first aspect of the present invention, there is provided an optical transmission system characterized in that: a plurality of Raman amplifiers including a plurality of pump light sources of different pump wavelengths are used; the longitudinal section of the optical fiber where stimulated Raman scattering is generated differs depending upon the plurality of Raman amplifiers; and the plurality of Raman amplifiers mutually compensate for the respective Raman gain wavelength properties.
That is, according to the first aspect of the present invention, there is provided an optical transmission system in which, at an output point for a wavelength division multiplexing signal of the optical transmission system using a plurality of stages of Raman amplifiers, the total Raman gain flatness of the plurality of Raman amplifiers smaller than the cumulative flatness on the assumption that every amplifier has the same gain profile of one of the plurality of Raman amplifier.
According to a second aspect of the present invention, in the first aspect of the invention, there is provided an optical transmission system characterized in that the plurality of Raman amplifiers are designed to be of at least two types of pump wavelength sets, and when there exist a plurality of Raman amplifiers of the same pump wavelengths which use the same kind of amplifier fibers, they include different set gains.
According to a third aspect of the present invention, in the first or second aspect of the invention, there is provided an optical transmission system characterized in that: at least a first Raman amplifier and a second Raman amplifier are used as the plurality of Raman amplifiers; a wavelength band where the Raman gain wavelength property of the first Raman amplifier exhibits an upward convex curve including a maximum value of Raman gain of GAmax at a wavelength of xcexAmax and a wavelength band where the Raman gain wavelength property of the second Raman amplifier exhibits a downward convex curve including a minimum value of Raman gain of GBmin at a wavelength of xcexBmin overlap with each other; a wavelength band where the Raman gain wavelength property of the first Raman amplifier exhibits an downward convex curve including a minimum value of Raman gain of GAmin at a wavelength of xcexAmin and a wavelength band where the Raman gain wavelength property of the second Raman amplifier exhibits an upward convex curve including a maximum value of Raman gain of GBmax at a wavelength of xcexBmax overlap with each other; and the total Raman gain flatness is smaller than the flatness of said first Raman amplifier and said second Raman amplifier.
That is, according to the third aspect of the invention, there is provided an optical transmission system in which, regarding the gain flatness of the optical transmission system using the first Raman amplifier and the second Raman amplifier, the maximum and minimum values of the respective Raman gain wavelength properties of the first Raman amplifier and the second Raman amplifier are not accumulated to cause an increase in the total Raman gain flatness, and in which the total Raman gain flatness can be set to be smaller than the gain flatness of each of the first Raman amplifier and the second Raman amplifier.
According to a fourth aspect of the present invention, in the third aspect of the invention, there is provided an optical transmission system characterized in that the wavelength xcexAmax and the wavelength xcexBmin are substantially equal to each other, and in which the wavelength xcexAmin and the wavelength xcexBmax are substantially equal to each other.
That is, according to the fourth aspect of the invention, there is provided an optical transmission system, in which the total Raman gain flatness of the optical transmission system using the first Raman amplifier and the second Raman amplifier is minimum.
According to a fifth aspect of the present invention, in any one of the first to fourth aspects of the invention, there is provided an optical transmission system characterized in that at least one of a discrete Raman amplifier and a distributed Raman amplifier is used as the Raman amplifier.
That is, according to the fifth aspect of the invention, there is provided an optical transmission system, in which discrete amplifiers or distributed amplifiers are used so that the total Raman gain flatness is smaller than the each Raman gain flatness of the plurality of Raman amplifiers constituting the optical transmission system.
For example, when laying a new line as a transmission path or when newly installing a Raman amplifier in an existing line where no Raman amplifier is used, the deviation of the gain wavelength property increases cumulatively if a large number of stages of Raman amplifiers having the same gain wavelength property are used when a large number of stages of Raman amplifiers are required, so that it is necessary to select either a discrete Raman amplifier or a distributed Raman amplifier for each Raman amplifier in order that the respective gain wavelength properties of the Raman amplifiers mutually compensate for so as to decrease the total gain flatness. When Raman amplifiers are to be added to an existing optical transmission system in which a Raman amplifier has already been installed, it is necessary to select either a discrete amplifier or a distributed amplifier for each of the additional Raman amplifiers in order that the gain wavelength property of the existing Raman amplifier and the gain wavelength property of the additional Raman amplifiers may mutually compensate for so as to decrease the total Raman gain flatness.
Here, the theory on which the present invention is based will be described.
There are two kinds of Raman amplifiers: a distributed amplifier and a discrete amplifier. In the distributed Raman amplifier, mainly the optical fiber transmission path itself is used as the medium for Raman amplification, so that, taking into account the distortion generated when, for example, amplifying the WDM signal, the optical fiber transmission path is one whose effective core area Aeff in the signal band wavelength is from approximately 50 xcexcm2 to 100 xcexcm2. It is not necessary for the actual gain of the distributed amplifier, that is, the Raman gain, to exceed the transmission loss of a relay section of the optical transmission path itself.
The discrete Raman amplifier is used as a relay amplifier, so that the effective core area Aeff in the signal band wavelength is made as small as approximately 10 xcexcm2 to 30 xcexcm2 to thereby increase the non-linearity, and, as far as the Raman gain due to the effective core area Aeff is concerned, it is so designed that the Raman gain of the discrete Raman amplifier is larger than the Raman gain of the distributed Raman amplifier by approximately 10 dB. Further, since the discrete Raman amplifier is used as a relay amplifier, a Raman gain which is in excess of the transmission loss of the highly non-linear optical fiber constituting the discrete Raman amplifier is required. That is, the gain of the discrete Raman amplifier must be a net gain, which is an apparent gain.
In the case of signal/wavelength and pump lightwave/wavelength, the Raman gain is approximately given as: G=exp(gRPOLeff/Aeff), where gR is the Raman gain coefficient; PO is the pump light power at the pump light input end of the optical fiber constituting the Raman amplifier; and Leff is the length (effective length) generating effective stimulated Raman effect. Assuming that xcex1p is the attenuation per unit length of the pump lightwave, Leff=(1/xcex1p)[1xe2x88x92exp(xe2x88x92xcex1pL)]. L is the length of the longitudinal section of the optical fiber where stimulated Raman scattering is generated by the Raman amplifier.
Thus, to experimentally obtain a Raman gain coefficient belonging to a desired wavelength band, a pump lightwave in which the pump light wavelength xcexp is fixed to the shortest wavelength side of the desired wavelength band is input to the pump light input end of the optical fiber constituting the Raman amplifier, and the signal light wavelength xcexs is swept from the short wavelength side to the long wavelength side of the desired wavelength band, measuring the pump light power PO at the pump light input end of the optical fiber constituting the Raman amplifier and the input end signal lightwaves input P1 and the output end signal lightwave output P2 of the Raman amplifier.
In correspondence with the fixed pump light wavelength xcexp and the swept signal light wavelength xcexs, the apparent gain (net gain) GN=10 log(P2/P1) is obtained from the input end signal lightwaves input P1 and the output end signal lightwaves output P2, and the signal light transmission loss xcex1gL which is the product of the attenuation xcex1s per unit length in the signal light wavelength of the optical fiber constituting the Raman amplifier by the length L of the optical fiber constituting the Raman amplifier, obtaining the actual gain (Raman gain) G from the equation: G=Gn+xcex1sL.
The pump light power PO at the pump light input end of the optical fiber constituting the Raman amplifier can be measured, and, since the length L of the optical fiber constituting the Raman amplifier is known, Leff can be obtained through calculation. Since the gain G has been obtained, gR/Aeff can be obtained from the equation: G=exp(gRPOLeff/Aeff), and since Aeff is known by a well-known experiment method, gR can be obtained. For gR to be clearly measured, it is necessary for the pump light power PO at the pump light input end of the optical fiber constituting the Raman amplifier to be approximately +10 dBm, and the signal lightwaves is required to be approximately xe2x88x9220 dBm.
Assuming that "ugr"p is the frequency corresponding to the pump light wavelength xcexp and that "ugr"s is the frequency corresponding to the signal light wavelength xcexs, the above-mentioned Raman gain coefficient gR("ugr"p, "ugr"s) is obtained with respect to the parameters ("ugr"p, "ugr"s) of the fixed pump light frequency "ugr"p and the signal frequency "ugr"s of the signal to be swept. It is theoretically known that the generalized Raman gain coefficient gR depends upon the frequency shift xcex94"ugr"="ugr"pxe2x88x92"ugr"s. Thus, in order to obtain the generalized Raman gain coefficient gR(xcex94"ugr"), gR(xcex94"ugr") of the same magnitude is brought into correspondence with gR("ugr"p, "ugr"s) corresponding to the parameters ("ugr"p, "ugr"s), and the parameters ("ugr"p, "ugr"s) are replaced by the frequency shift xcex94"ugr"="ugr"pxe2x88x92"ugr"s.
In this way, the wavelength (frequency) dependency of the generalized Raman gain coefficient (or gR/Aeff) is obtained to prepare a table of wavelength (frequency)/Raman gain coefficient (or gR/Aeff).
The basic idea regarding the way pump lightwave, signal lightwaves, and optical fiber attenuation are related with each other in Raman amplification is described in xe2x80x9cPump Interactions in a 100 nm Bandwidth Raman Amplifierxe2x80x9d IEEE PHOTONICS TECHNOLOGY LETTERS, VOL.11, No. 5, MAY 1999, p.530.
Based on this idea, a basic equation showing how pump as lightwave, signal lightwaves, and optical fiber attenuation for Raman amplifier simulation are related was obtained on the assumption that the pump lightwave and signal lightwaves advance, that Rayleigh scattering is small, and that the influence of spontaneous emission light is negligible.
Assuming that the optical fiber attenuation at frequency "ugr" is xcex1"ugr", that the effective core area is Aeff"ugr", and that the Raman gain coefficient between frequency "xgr" and frequency "ugr" is gR("xgr"xe2x88x92"ugr")=g"xgr""ugr", the change in power at the time when advancing wave of power P"ugr" at frequency "ugr" has advanced by distance Z is expressed by the following equation:                                           ⅆ                          P              v                                            ⅆ            z                          =                                            -                              α                v                                      ⁢                          P              v                                +                                    P              v                        ⁢                                          ∫                                  c                   greater than                   v                                            ⁢                                                                    g                    cv                                                        A                                          eff                      ⁢                                              xe2x80x83                                            ⁢                                                                                            ⁢                                  P                                                  ⁢                                  ⅆ                                                                                -                                    P              v                        ⁢                                          ∫                                  c                   less than                   v                                            ⁢                                                                    g                                          v                      ⁢                                              xe2x80x83                                            ⁢                                                                                                  A                    effv                                                  ⁢                                  P                                                  ⁢                                  ⅆ                                                                                                          Equation        ⁢                  xe2x80x83                ⁢        1            
It is assumed here that the powers of WDM signals at frequencies "ugr"1, "ugr"2, . . . , "ugr"n are Ps"ugr"1, Ps"ugr"2, . . . , Ps"ugr"n, that the powers of multiplexing pump lightwaves at frequencies "xgr"1, "xgr"2, . . . , "xgr"m are Pp"xgr"1, Pp"xgr"2, . . . , Pp"xgr"m ("ugr"1 less than "ugr"2 less than . . .  less than "ugr"n less than "xgr"1 less than "xgr"2 less than . . .  less than "xgr"m), and that there is no Raman transition between the WDM signals.
As stated above, both WDM signal and pump lightwave are discrete spectrums, so that the integration of equation 1 is replaced by addition.
By applying a WDM signal of a frequency "ugr"k and power Ps"ugr"k to P"ugr"of equation 1, the following equation is obtained:                                           ⅆ                          P              svk                                            ⅆ            z                          =                                            -                              α                vk                                      ⁢                          P              svk                                +                                    P              svk                        ⁢                                          ∑                1                gn                            ⁢                              xe2x80x83                            ⁢                                                                    g                                                                ⁢                                              xe2x80x83                                            ⁢                      vk                                                                            A                                          eff                      ⁢                                              xe2x80x83                                            ⁢                                                                                            ⁢                                  P                                      p                    ⁢                                          xe2x80x83                                        ⁢                                                                                                                              Equation        ⁢                  xe2x80x83                ⁢        2            
Further, by applying a pump lightwave of a frequency "xgr"j and power PP"xgr"j to P"ugr"of equation 1, the following equation is obtained:                                           ⅆ                          P              pq                                            ⅆ            z                          =                                            -                              o                ci                                      ⁢                          P              rcj                                +                                    P              pcj                        ⁢                                          ∑                1                gn                            ⁢                              xe2x80x83                            ⁢                                                                    g                                                                ⁢                                              xe2x80x83                                            ⁢                      ci                                                                            A                                          eff                      ⁢                                              xe2x80x83                                            ⁢                                                                                            ⁢                                  P                                      p                    ⁢                                          xe2x80x83                                        ⁢                                                                                                    -                                    P              pci                        ⁢                                          ∑                1                                                      ς                    ⁢                                          xe2x80x83                                        ⁢                    i                                    -                  1                                            ⁢                              xe2x80x83                            ⁢                                                                    g                                          ci                      ⁢                                              xe2x80x83                                            ⁢                                                                                                  A                                          eff                      ⁢                                              xe2x80x83                                            ⁢                                            ⁢                                              xe2x80x83                                            ⁢                      i                                                                      ⁢                                                                          i                                                                    ⁢                                  P                                      p                    ⁢                                          xe2x80x83                                        ⁢                                                                                                    -                                    P              pci                        ⁢                                          ∑                1                vi                            ⁢                              xe2x80x83                            ⁢                                                                    g                                                                ⁢                                              xe2x80x83                                            ⁢                      iv                                                                            A                                          eff                      ⁢                                              xe2x80x83                                            ⁢                                            ⁢                                              xe2x80x83                                            ⁢                      i                                                                      ⁢                                                                          i                                    v                                ⁢                                  P                  sv                                                                                        Equation        ⁢                  xe2x80x83                ⁢        3            
The power (Ps"ugr"1, Ps"ugr"2, . . . , Ps"ugr"n) of the WDM signal is given at the signal input point, and the multiplexing pump lightwave (Pp"xgr"1, Pp"xgr"2, . . . , Pp"xgr"m) is given at the pump lightwave input point, so that the following results are achieved in the cases of forward pump, backward pump, and a combination of both.
First, the case of forward pump will be described. By substituting the initial value set when Z=0(Ps"ugr"1, Ps"ugr"2, . . . , Ps"ugr"n) and (Pp"xgr"1, Pp"xgr"2, . . . , Pp"xgr"m), respectively into the right side of equation 2 and equation 3, xcex94Ps"ugr"k and xcex94Pp"xgr"j spaced apart from Ps"ugr"k and Pp"xgr"j by xcex94z with respect to arbitrary "ugr"k and arbitrary "xgr"j are obtained. By substituting Ps"ugr"k+xcex94Ps"ugr"k and Pp"xgr"j+xcex94Pp"xgr"j respectively into the right sides of equation 2 and equation 3 as Ps"ugr"k and Pp"xgr"j, it is possible to obtain Ps"ugr"k and Pp"xgr"j spaced apart from the point where z=0 by 2xcex94z. By repeating this, it is possible to obtain (Ps"ugr"1, Ps"ugr"2, . . . , Ps"ugr"n) and (Pp"xgr"1, Pp"xgr"2, . . . , Pp"xgr"m) at the output point. Thus, the gain is known.
Next, the case of backward pump will be described. It is assumed that multiplexing pump lightwave (Pp"xgr"1, Pp"xgr"2, . . . , Pp"xgr"m) is subjected to backward pump when Z=L, and that the anticipated value of multiplexing pump light power when Z=0 is Pp"xgr"kxe2x88x92xcex1"xgr"kL (k is an integer ranging from 1 to m), taking into account the transmission loss xcex1"xgr"k. The initial value of the WDM signal lightwaves (Ps"ugr"1, Ps"ugr"2, . . . , Ps"ugr"n) when Z=0 and the anticipated value of multiplexing pump light power are computed in the same manner as in the case of forward pump. In this process, equation 2 is used as it is, and, regarding equation 3, + sign and xe2x88x92 sign are exchanged, obtaining the multiplexing pump light power (Pp"xgr"1, Pp"xgr"2, . . . , Pp"xgr"m) when Z=L. When the computed multiplexing pump light power and the set multiplexing pump light power coincide with each other within the set error range, the computed WDM signal lightwaves (Ps"ugr"1, Ps"ugr"2, . . . , Ps"ugr"n) when Z=L is adopted as a solution satisfying equation 2 and a solution satisfying the equation in which the signs of equation 3 are exchanged. When the computed multiplexing pump light power and the set multiplexing pump light power are outside the set error range, the magnitude of the multiplexing pump light power (Pp"xgr"1, Pp"xgr"2, . . . , Pp"xgr"m) when Z=0 is anticipated and set anew, and the initial value of the WDM signal lightwaves (Ps"ugr"1, Ps"ugr"2, Ps"ugr"n) when Z=0 and the anticipated set value of the multiplexing pump light power are, as in the case of forward pump, substituted into equation 2 and the equation obtained by exchanging the signs of equation 3, obtaining the multiplexing pump light power (Pp"xgr"1, Pp"xgr"2, . . . , Pp"xgr"m) when Z=L. The above computation is repeated until the computed multiplexing pump light power when Z=L and the set multiplexing pump light power coincide with each other within the set error range.
Next, the case of a combination of forward pump and backward pump will be described. Here, it is assumed there is no mutual action between forward pump and backward pump. Thus, superposition of forward pump and backward pump is performed to obtain gain. That is, the sum of equation 3 indicating the pump lightwave at the time of forward pump and an equation obtained by exchanging the signs of equation 3 indicating the pump lightwave at the time of backward pump is substituted into the Pp"xgr" of equation 2 indicating the pump lightwave, repeating the algorithm for forward pump and backward pump.
As in the above three cases, simulation is performed using equation 2 and equation 3, the gain wavelength properties of two Raman amplifiers compensate for each other, and a value at which the total Raman gain flatness is decreased is searched for. By applying the value to an actual system, it is possible to realize the optical transmission system of the present invention.